Re: GBlist: Re: mg additive for cement?

Marc J. Rosenbaum (Marc.J.Rosenbaum@valley.net)
05 Mar 97 08:07:30 EST

The right question to ask, I think, is - do you *need* higher thermal
conductivity? If the issue is storing heat on a daily basis from passive solar
gain, it seems that we know that the first four or so inches of mass does the
work. The reason that Mazria mentions high conductivity brick is that brick
has only about 40% of the thermal conductivity of concrete (source: Passive
Solar Design Handbook, Vol. II - LASL 1980). Concrete appears to be conductive
enough in normal (4 inch) thicknesses to not require augmentation of its
conductivity.

As for increasing conductivity to use less concrete: the thinner the layer of
concrete, the less the thermal conductivity matters. The PSD Handbook gives
the following values for energy stored in concrete on a daily basis in a
passive solar wall directly irradiated by the sun:

1" - 2.5 BTU/ft2-F
2" - 4.99
3" - 7.37
4" - 9.47
6" - 11.94
8" - 12.14

You can see that it is darn linear to 4 inches! So increasing conductivity
wouldn't make sense until you got to an eight inch wall.

Marc Rosenbaum
__________________________________________________________________
This greenbuilding dialogue is sponsored by Oikos (www.oikos.com)
and Environmental Building News (www.ebuild.com). For instructions
send e-mail to greenbuilding-request@crest.org.
__________________________________________________________________